Distributed RFID antenna array utilizing circular polarized helical antennas

ABSTRACT

In accordance with the teachings described herein, RFID systems are provided that include a distributed RFID antenna array utilizing one or more circular polarized helical antennas. A plurality of RFID tags may be used, with each RFID tag including a linear polarized antenna for communicating RFID tag signals. One or more receiver antennas may be used for receiving the RFID tag signals from the RFID tags. An RFID tag signal reader may be used to process RFID tag signals received by the receiver antennas. In one example, the receiver antennas may include a circular polarized helical antenna element. One or more transmitter antennas may be used for transmitting an RF signal to the plurality of RFID tags, the transmitter antennas including a circular polarized helical antenna element. A transmitter may be used to generate the RF signal for transmission by the transmitter antennas. In one example, the RFID tag signal reader and the transmitter may be included in a single reader/transmitter unit.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.11/417,768, filed on May 4, 2006, which is a continuation-in-part ofInternational Patent Application No. PCT/US05/37138, filed on Oct. 18,2005, which claims priority from U.S. Provisional Application No.60/625,273, filed on Nov. 5, 2004. These prior applications areincorporated herein by reference in their entirety.

FIELD

The technology described in this patent document relates generally toradio frequency identification (RFID) systems. More particularly, thepatent document describes a distributed RFID antenna array that utilizesone or more circular polarized helical antennas.

BACKGROUND

The RFID system described herein is related to the inventions describedin commonly assigned U.S. Patent Application Pub. No. 2004/0056091,which is incorporated herein by reference in its entirety. In thatpatent application, it was pointed out that a need exists for anadvertising compliance monitoring system that provides versatility andflexibility by providing an RFID tag, associated with a specific sign orproduct display, that communicates tag data to an external reader.

U.S. Patent Application Pub. No. 2004/0056091 describes an RFID systemthat may include RFID tags of various types (e.g., passive, semi-passiveor active), backscatter reader transmitters (BRT), and hubs. Typically,each BRT is a fully self-contained, battery operated unit, and utilizesthree antennas. Two medium-gain patch antennas are used to read thetags, and a whip antenna is used to report the received data over awireless link to the hub. This system functions well and is capable ofdetecting and reporting tags in a variety of retail environments and atdifferent frequencies. It is desirable, however, to provide an even moreeconomical RFID system by centralizing some or all of the electronicsthat have been distributed across areas or sub-areas in a givenfacility, thereby reducing redundancy and cost. It is also desirable toincrease the read range of tags by the system to reduce the number ofantennas required and to increase the reliability of tags being readunder marginal conditions.

SUMMARY

In accordance with the teachings described herein, RFID systems areprovided that include a distributed RFID antenna array utilizing one ormore circular polarized helical antennas. A plurality of RFID tags maybe used, with each RFID tag including a linear polarized antenna forcommunicating RFID tag signals. One or more receiver antennas may beused for receiving the RFID tag signals from the RFID tags. An RFID tagsignal reader may be used to process RFID tag signals received by thereceiver antennas. In one example, the receiver antennas may include acircular polarized helical antenna element. One or more transmitterantennas may be used for transmitting an RF signal to the plurality ofRFID tags, the transmitter antennas including a circular polarizedhelical antenna element. A transmitter may be used to generate the RFsignal for transmission by the transmitter antennas. In one example, theRFID tag signal reader and the transmitter may be included in a singlereader/transmitter unit.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts an example RFID system that includes a BRT hub thatcovers a designated area such as an entire commercial sales facility.

FIG. 2 depicts an example RFID system that includes a plurality of BRThubs that are used in a plurality of designated areas to cover a largerfacility.

FIG. 3 depicts an example RF transmitter with a high power amplifier anda band-pass filter.

FIG. 4 depicts an object having an RFID tag associated therewith.

FIG. 5 is a graph illustrating example quadrifiler helix antenna gainpatterns to show that the antenna has a low gain on the axis and a highgain on the sides.

FIG. 6 depicts an example switched backscatter tag (SBT) illustratingthe manner in which the switch is opened and closed to accept or rejecta BRT carrier signal.

FIGS. 7A and 7B depict an example transmitter antenna having a circularpolarized quadrifiler helix antenna element.

FIG. 8 depicts the example quadrifiler helix antenna attached to anamplifier circuit.

FIGS. 9A and 9B depict an example receiver antenna having a single turnhelix antenna element.

FIG. 10 depicts the example single turn helix antenna attached to anamplifier circuit.

FIGS. 11-13 depict another receiver antenna embodiment that includes asingle turn helix antenna element.

DETAILED DESCRIPTION

FIG. 1 depicts an example RFID system that includes a backscatterreader/transmitter (BRT) hub (called a “Spider”) that covers adesignated area of a facility. The RFID system may, for example, be usedto detect and report the presence and location of radio frequency (RF)tags across selected zones in a retail environment. The RFID system mayalso be used to centralize RF transmission and receiving functions toreduce the expense of recurring components. A single BRT hub (“Spider”)may be used that includes antennas attached to multiple transmit andreceive ports to cover a designated area of a facility. In smallfacilities, a single BRT hub may be used to cover the entire facility asthe designated area. The Spider may, for example, be connected to ACpower to eliminate the cost and maintenance of batteries, as well asallowing more read cycles, if desired. This also may permit higherwattage to be used in the transmit function, potentially increasing thesize and reliability of detection zones.

In FIG. 1, a small facility 10 is shown in which the designated area 12to be covered by a BRT hub 14 includes the entire facility. The BRT hub14 is coupled to a plurality of transmitters (TX 1, 2) 16-18 and aplurality of receivers (RX 1-10) 20-38, for example using coaxial cable.The plurality of receivers 20-38 are positioned to provide coverage ofthe entire designated area 12 (the entire facility 10). Preferably, onlyone TX and one RX are active at a time. It will be noted that RX 22 isable to receive data from RFID tags 60, 62, and 64 at differentdistances in the sub-area covered by RX 22, as illustrated by concentriccircles 54, 56, and 58. Also it will be noted that the transmitter TX 16has concentric rings 48, 50, and 52 that illustrate thetransmitter-to-tag zones covered by the range of transmitter TX 16, thusshowing that the transmitting antenna TX 16 is positioned to illuminateat least a portion of the RFID tags (in the RX zones covered by RX 20,22, 26, 30, 34, 36, and 38) in the designated area. In like manner, TX18 shows corresponding concentric rings illustrating illuminationcoverage ranges and representing transmitter-to-tag zones covering atleast a portion of the RFID tags. Between the two transmitters TX 16 and18, all of the RFID tags in the designated area (the facility 12) arecapable of illumination.

Each of the transmitters TX 16 and 18 is coupled to the BRT hub 14, forexample with coaxial cable. In like manner, each of the receiverantennas in each sub-area is coupled to the BRT hub 14, for exampleusing coaxial cable. Of course, wireless connections, or otherwell-known known types of connections could be used instead of coaxialcable.

When the transmitting antenna 16 illuminates RFID tags within its range,one of the RF signal receiving antennas, such as RX 22, receives themodulated tag signals and conveys them to the BRT hub 14 over coaxialcable (such as 42) for transmission to a remote server. A modulated RFIDtag signal may be received by more than one RX antenna when readsequentially (for example RX 26 and RX 28). In such cases, the BRT hub(Spider 14) may forward both RX events to the server, and may ascertaina location within a store using closest zone readings, received signalstrength indicator (RSSI) readings, antenna intersection, or otheralgorithms. One preferred method is disclosed in commonly assignedcopending application Ser. No. 11/418,319, entitled “Systems and Methodsfor Approximating the Location of an RFID Tag,” filed on even dateherewith, the subject matter of which is incorporated herein in full.

The transmitting antennas 44 and 46 associated with respectivetransmitters TX 16 and 18 should be omni-directional in order toilluminate tags over a large area. A shaped beam with low gain on axisand a high gain to the sides is ideal. For example, a quadrifiler helixantenna, as illustrated in FIGS. 7 and 8, may be used for thetransmitting antennas 44 and 46. Quadrifiler helix antennas have beenthe choice in orbiting spacecraft communications for years. Aquadrifiler helix antenna has circular polarization and a shaped beamfor high gain when the spacecraft is farthest away on the earth'shorizon, and low gain when the spacecraft is closest or overhead. Also,when used in an RFID system as described herein, the low profile of anquadrifiler antenna is equally advantageous. To a consumer or otherobserver in the facility, a quadrifiler helix antenna will typicallylook like a small white paper towel tube that hangs down a few inchesvertically from the ceiling.

Typically, the transmit beam gain from TX 16 to RX 38 would be lowerthan the transmit beam gain from TX 16 to RX 22. Quadrifiler helixantennas are range compensating. The gain of the antenna is higher forobjects farther away, which compensates for free-space power loss due todistance. This is illustrated in FIG. 5 which shows power vs. antennaangle. Higher power levels (gain) at 70 degrees are offset by the boresight of the antenna.

Further, quadrifiler helix antennas are typically inexpensive. Theantennas 44 and 46 shown in FIG. 1, for example, may be constructed ofmaterials, such as PVC piping, #12 copper wire, and a small circuit cardto maintain proper phasing between the elements. This type of antennahas been experimentally tested in a retail environment with verysuccessful results.

Under FCC rules, part 15, a conducted RF output power of 1 Watt isallowed. The BRT's that are used in the system disclosed in commonlyassigned U.S. Patent Application Publication No. 2004/0056091 arebattery powered and have a maximum output power of 200 mW to conservebattery life while “illuminating” tags (e.g., reflect and receivebackscatter modulated signals produced by the tags). Increasingconducted transmitter power will illuminate tags in a larger area andbetter illuminate tags marginally located in existing zones. The use ofthe quadrifiler helix antenna enables a gain of approximately 6 dbictranslating into an effective isotropic radiated power (EIRP) of +36 dBmor 4 W. This is an increase of approximately 9 dB over the BRT patchantenna disclosed in the above identified published and commonlyassigned co-pending patent application. This translates into an increaseof 8 times the power.

The performance of an RF reader may be affected by transmitter powerbeing coupled into the BRT receiver through the receiver antenna. Thebackscattered signal from the RFID tag is extremely small, and itsdetection can easily be overwhelmed by the backscatter transmittercarrier wave signal. Therefore, the separation of the TX antenna and theRX antenna, as shown in FIG. 1, improves performance because thedeployment system allows for excellent separation.

Also, the use of the switched backscatter RFID tag (SBT) 102 shown inFIG. 6 also improves the signal communications between the SBT and theBRT. In one example, the SBT 102 has an antenna in which each side 104and 106 of the antenna is approximately ¼λ (i.e., ¼ wavelength). In thecase of a 915 MHz tag, each side is about 3.2 inches long. For a 2.45GHz tag, these lengths would be approximately 1.2 inches long. Thus, fordifferent frequencies the antenna lengths also would be different. Abackscatter generator 110 produces a sub-carrier frequency that containsdata, such as a tag ID. This backscatter signal opens and closes the RFswitch 108 that connects the resonant ¼λ antenna elements 104 and 106.When the switch 108 is in the closed position, the antenna acts as a ½λelement, which is not a good receiver, and that reflects a higherpercentage of the reader carrier frequency back to the reader.

When the switch 108 is in the open position, as shown, each antenna sideis ¼ of the wavelength of the carrier frequency, which makes it a goodreceiver, and therefore absorbs more of the reader carrier frequency soit is not reflected back to the reader. This combination results in asubstantial increase in the ratio of a “mark” (a 1 in binary statemonitoring) to “space” (a 0 in binary state monitoring) signal receivedby the BRT. The increased ratio results in a dramatic improvement in thereader's ability to track the modulated signal containing the tag dataacross much larger distances. It also allows tags to be read more easilyunder marginal conditions, such as when they are close to liquid ormetal (conditions well known in the art to be quite challenging for tagsin the UHF band). In one example, the tag has improved performancebecause the antenna is T-shaped, with the antenna elements across thetop of the tag, pointing out and away from other circuitry on theprinted circuit board. This increases the effectiveness of the availablefrequency aperture and reduces antenna de-tuning.

The clean switching between “on” and “off” of a resonant apertureincreases the mark-to-space ratio of the backscatter data as received bythe BRT. It is this increased ratio that improves the BRT's ability todetect tags in a specific area of the store area being monitored using acarrier frequency, thereby allowing tags with a cleanly-switchedresonant aperture to be detected at a much greater distance than tagswithout a cleanly-switched resonant aperture.

The system shown in FIG. 1 is well-suited for a small commercial salesestablishment, such as a drug store, but a single Spider would likely beinsufficient for larger-format retailers, such as grocery or massmerchandiser outlets. In such cases, several Spiders, each with separateWebs, could be used to cover the establishment. Connectivity to phonelines and redundant external communication electronics across multipleSpiders in a store could be circumvented by centralizing those functionsinto one master Spider 84. Such a system is shown in FIG. 2.

Note in FIG. 2 that the selected location, or retail sales facility 10,is too large for one Spider. Therefore, in this example, four designatedareas 72, 74, 76, and 78 are used to cover the entire facility 10. Eachof the systems in each of the designated areas 72-78 is identical to thesystem shown in FIG. 1 and operates in an identical manner as describedabove. However, each of the Spiders 80, 82, 84, and 86 could beelectronically coupled to a master hub 88 as shown.

Multiple Web antennae are connected to a single backscattertransmitter/receiver in the Spider, for example through coaxial cables.These coaxial cables pass through a switch matrix. This matrix and thelong coaxial cables combine to create additional attenuation, therebylowering the received signal level. To overcome this loss, a low noiseamplifier (LNA) is positioned at each RX antenna. These amplifiers drawsmall amount of current (≈15 mA) through the coaxial cable using biastees. Locations in retail environments that are difficult or expensiveto monitor via coaxial cable, such as external fuel pump signage, couldstill be served by the previously-designed BRT's with distributedreader/transmitter electronics by forwarding their data wirelessly tothe master Spider.

FIG. 3 is a block diagram of an example quadrifiler helix antenna 90.The antenna 90 is coupled to the Spider through a coaxial cable 92 andhas an associated high power amplifier 94 to recover coaxial cablesignal attenuation. The antenna 90 also has an associated ISM(Industrial, Scientific, and Medical) band pass filter 96 to reducenoise or harmonics.

FIG. 4 depicts an example object 98 having an PFID tag 100 associatedtherewith. The object may be a permanent display, Point of Purchase(POP) temporary display, signage, advertising material, stock-alertsensors, merchandising material, category section marker, individualproduct, or other material desired to be monitored by retailers,manufacturers, or point-of-sale producers (collectively a “display”).The object may also be a consumer (or movable object) to which an RFIDtag is associated so that the shopping (movement) pattern of theconsumer can be monitored. In this manner, consumer exposure to a givendisplay may be tracked. An RFID tag given to a consumer may, forexample, be a small active transmitter tag (ATT) that uses the samefrequency and protocol as the reflection from the semi-passivebackscatter tags.

FIGS. 7-10 depict example circular polarized antenna configurations thatmay be used as transmitter and receiver antennas in an RFID system, asdescribed herein. It has been determined that for both economic andperformance reasons the optimal solution for the antennas in an RFIDsystem is to use circular polarized antennas for the transmitters andreceivers and to use linear polarized antennas for the RFID tags. Theswitched backscatter RFID tag (SBT), described herein, is one example ofan RFID tag having a linear polarized antenna.

Using a linear polarized tag in an RFID system is typically moreeconomical than using a tag with circular polarization. A linearpolarized tag can typically be made smaller than a tag using circularpolarization because a linear polarized antenna needs to operate in onlyone axis. However, from a system standpoint the radiation patterns ofthe antennas in the transmitter, receiver and tag should all be alignedor coplanar to achieve the most robust link and the best performance.This is most easily achieved in a retail environment using circularpolarized antennas because maintaining coplanar antenna alignmentbetween linear antennas in a retail environment is often impractical. Agood compromise is the use of circular polarized antennas for thereceivers and transmitters and linear polarized antennas for the RFIDtags. In this manner, a high level of overall system performance may bemaintained, while reducing the cost of the RFID tags.

FIGS. 7A and 7B depict an example transmitter antenna 200 that includesa quadrifiler helix antenna element 202. FIG. 7A is a side view of theantenna structure 200 and FIG. 7B is an exploded view in which theantenna element 202 and dielectric core 204 are depicted separately. Thedielectric core 204 is a cylindrical structure formed from anon-conducting material. The antenna element 202 includes four radiatingarms that are joined at a common junction 206 and that extend from thecommon junction in a helical pattern. In one example, the antennaelement 202 may be formed from two antenna wires that are joined at thecommon junction 206, for instance by soldering, and that are shaped toform the four radiating arms of the quadrifiler helix structure. Inanother example, the two wires forming the antenna element may be inphysical contact, but not mechanically joined, at the common junction206.

In the illustrated example, the antenna structure 202 is attached to thedielectric core 204 using a plurality of holes 208 in the dielectriccore 204. As illustrated in FIG. 7A, the antenna structure 202 may beattached through the holes 208 in the dielectric core 204, such that thecommon junction 206 is within the cylinder of the core 204 and thespiral portions of the radiating arms extend through an upper set ofholes 208 and along the outside of the dielectric core 204. The fourradiating arms may also extend through a lower set of holes 208 suchthat the four end portions 210 of the radiating arms extend from insideof the dielectric core 204. In addition, the antenna element 202 may befurther secured to the dielectric core 204, as well as protected fromenvironmental conditions, by covering the radiating arms on the outsideof the core 204 with a protective material, such as a heat shrink, asshown in FIG. 7A.

FIG. 8 depicts the example quadrifiler helix antenna 200 attached to anamplifier circuit 220. As illustrated, the end portions 210 of theantenna element 202 may extend through a dielectric material 222, suchas a printed circuit board, to couple the antenna 202 to the amplifiercircuit 220. The dielectric material 222 may also incorporate an antennabackplane (e.g., a metallic surface) to shield the antenna 202 from theamplifier circuit 220 and to provide directivity to the circularpolarized radiation pattern of the helical antenna element 202.

The amplifier circuit 220 may, for example, be attached to the ceilingof a retail environment such that the antenna 200 extends downwardlyfrom the ceiling. In addition, the amplifier circuit 220 may be coupledto other components in the RFID system via an external connector 224,such as a coaxial cable connector. In one example, the amplifier circuit220 may include two or more gain settings that may be used to tune theamplifier circuit 220 for use in different sized retail environments.For example, a higher gain setting for the amplifier 220 may be used fora larger retail environment.

FIGS. 9A and 9B depict an example receiver antenna 230 that includes asingle turn helix antenna element 232. FIG. 9A is a prospective view ofthe antenna structure 230 showing both the antenna element 232 and thedielectric core 234, and FIG. 9B shows only the antenna element 232. Thedielectric core 234 is a cylindrical structure formed from anon-conducting material. In the illustrated example, the antenna element232 is attached to the dielectric structure 234 using a hole 236 in abottom portion of the dielectric core 234 and a slot 238 in an upperportion of the core 234. As illustrated in FIG. 9A, an upper end portion240 of the antenna element 232 may extend trough the slot 238 and alower end portion 242 of the antenna element 232 may extend through thehole 236, such that the spiral portion of the antenna element extendsalong the outside of the dielectric core 234.

FIG. 10 depicts the example single turn helix antenna 230 attached to anamplifier circuit 250. As illustrated, the lower end portion 242 of theantenna element 232 may extend through a dielectric material 252, suchas a printed circuit board, to couple the antenna 232 to the amplifiercircuit 250. The dielectric material 252 may also incorporate an antennabackplane (e.g., a metallic surface) to shield the antenna 232 from theamplifier circuit 250 and to provide directivity to the circularpolarized radiation pattern of the helical antenna structure 232. FIG.10 also illustrates a conductive patch 245 that may be included to tunethe antenna and possibly to help adhere the antenna element 232 to thedielectric material 252. The element 232 may be adhered to the outsideof the patch 245.

The amplifier circuit 250 may, for example, be located in the ceiling ofa retail environment, for example above the ceiling tiles. In addition,the amplifier circuit 250 may be coupled to other components in the RFIDsystem via an external connector 254, such as a coaxial cable connector.

FIG. 11 depicts another preferred embodiment of receiver antenna 300that includes a single turn helix antenna element 302. In this example,the antenna element 302 is not supported by a dielectric core. Rather,the antenna element 302 is attached to a dielectric material 304, suchas a printed circuit board, using a plurality of support structures 306made of a dielectric material, such as plastic. In addition, an endportion of the antenna element 302 is coupled to an amplifier circuit310 through a hole 308 in the dielectric material 304. The dielectricmaterial 304 may also incorporate an antenna backplane (e.g., a metallicsurface) to shield the antenna element 302 from the amplifier circuit310 and to provide directivity to the circular polarized radiationpattern of the helical antenna structure 302. Also illustrated is aconnector 312, such as a coaxial cable connector, for coupling theamplifier circuit 310 to other components in the RFID system. In oneexample, the antenna element 302 may be about 1λ in length with a pitchof about 0.2λ The openings 307 in the supports 306 serve to fix thepitch at the beginning portion of the element 302 at its criticalbeginning location.

FIG. 12 is an exploded view of an example enclosure 330, 335 for housingthe receiver antenna 300. The antenna housing 330, 335 may, for example,be secured in the ceiling of a retail environment, for example above theceiling tiles. FIG. 13 shows how the antenna structure 302 fits withinthe housing portions 330, 335.

This written description uses examples to disclose the invention,including the best mode, and also to enable a person skilled in the artto make and use the invention. The patentable scope of the invention mayinclude other examples that occur to those skilled in the art.

1. A coreless helical antenna structure, comprising: a dielectric basestructure; an antenna element extending away from the dielectric basestructure in a spiral pattern and having a circular polarized radiationpattern; and a plurality of support structures that attach the antennaelement to the dielectric base structure.
 2. The coreless helicalantenna structure of claim 1, wherein the plurality of supportstructures are configured to maintain the spiral pattern of the antennaelement.
 3. The coreless helical antenna structure of claim 1, whereinthe antenna element includes a single radiating arm that extends awayfrom the dielectric base structure in the spiral pattern.
 4. Thecoreless helical antenna structure of claim 3, wherein the singleradiating arm of the antenna element forms a single turn helix antenna.5. The coreless helical antenna structure of claim 3, wherein the singleradiating arm is formed from a single antenna wire.
 6. The corelesshelical antenna structure of claim 1, further comprising: a metallicantenna backplane that adds directivity to the circular polarizedradiation pattern of the antenna element.
 7. The coreless helicalantenna structure of claim 6, wherein the metallic antenna backplane isintegral to the dielectric base structure.
 8. The coreless helicalantenna structure of claim 1, wherein the plurality of supportstructures include openings and the antenna element is supported withinthe openings.
 9. The coreless helical antenna structure of claim 3,wherein the plurality of support structures maintain a desired pitch ofthe antenna element with respect to the dielectric base structure. 10.The coreless helical antenna structure of claim 9, wherein the antennaelement has a total length and the pitch is about equal to ⅕ the totallength of the antenna element.
 11. The coreless helical antennastructure of claim 1, wherein the plurality of support structures aremade of a dielectric material.
 12. The coreless helical antennastructure of claim 11, wherein the plurality of support structures areplastic.
 13. The coreless helical antenna structure of claim 1, furthercomprising an amplifier circuit coupled to the antenna element andoperable to amplify a signal received by the antenna element.
 14. Thecoreless helical antenna structure of claim 1, wherein the corelesshelical antenna structure is configured as a receiver antenna for anRFID system.
 15. The coreless helical antenna structure of claim 1,wherein the coreless helical antenna structure is supported within anenclosure.
 16. A coreless helical antenna structure, comprising: adielectric base structure; an antenna element that includes a singleradiating arm extending away from the dielectric base structure in aspiral pattern and having a circular polarized radiation pattern; and asupport structure that attaches the antenna element to the dielectricbase structure and that is configured to maintain a desired pitch of theantenna element with respect to the dielectric base structure.